In order to detect a crank angular position of the engine, a ring gear is fixed to the crankshaft, and has plural projections (teeth) formed at equal intervals as well as one toothless portion provided to have an irregular pitch to determine the reference position of the crankshaft. Determining the projections and recesses while the engine is rotating to identify the toothless portion, that is, a reference position by means of a ratio between the projections and the recesses allows detection of the crank angle. Fuel injection timing and ignition timing are controlled based on the detected crank angle.
In this case, when each tooth passes a crank angle sensor located facing an outer periphery of the ring gear due to the engine rotation, the crank angle sensor detects a lateral length of the tooth as well as an interval of the two adjacent teeth to calculate the ratio therebetween. This allows distinguishing of the teeth positioned at regular pitch from the toothless portion, thereby detecting the reference position.
However, since each tooth is detected when passing the crank angle sensor with the engine rotation, unstable engine speed may cause lower detection accuracy. A four-cycle engine makes two rotations to complete one cycle consisting of intake, compression, expansion and exhaust. In one cycle described above, the fluctuations in the engine speed repeat the increase in the expansion stroke, and then gradual decrease in the exhaust, intake and compression strokes. In particular, restarting the engine after it had a rest results in greater fluctuation in engine rotational speed.
A crank angular position is detected by detecting a projection (tooth's lateral length) and a recess (interval of the two adjacent teeth) of each tooth provided on the ring gear of the crankshaft, and by distinguishing the toothless portion based on the ratio therebetween. In this case, the ratio between the lateral lengths of tooth's projection and recess varies depending on the fluctuations in engine rotational speed, which may result in incorrect determination of the toothless portion, that is, a reference crank angle position. This requires adequate time for more reliable determination of the reference crank angle position.
An advantage of the present invention made in view of the prior art described above is to provide a crank angle detecting device for an engine independent of the fluctuations in engine rotational speed while performing more reliable detection of the reference crank angle position.